There are many uses for high pressure control valves, including controlling flow of gas, steam, water and the like to compensate for load disturbances and regulate process variables within a control loop. Modern high-pressure control valves use low-noise trim to enable high pressure gases and liquids to flow without excessive noise and to maintain a desired flow coefficient (Cv). Valve plugs used to modulate the flow rate under high pressure and changing pressure conditions include globe valves that use either a seat ring trim or a cage trim. A globe valve with an integral seat ring and an unbalanced valve plug is generally chosen for smaller valve sizes. In contradistinction, larger valve sizes, in order to be pressure balanced and provide for low noise, generally incorporate cage-type trim.
There are significant reasons to prefer a seat ring type trim to a cage-type trim for a control valve. For example, globe valves with a seat ring trim are lower in cost, and do not present thermal expansion problems. These valves provide better alignment of the valve plug with the valve seat and require only one gasket. Valves with seat ring trim can also incorporate a skirt that at least partially obstructs fluid flow, reducing the amount of flow in a fully open valve. In a worst case, a skirt can produce vortices, turbulence and pressure gradients causing hydrodynamic plug forces and cavitations. From the laws of fluid mechanics, it is known that when a fluid discharges from an orifice into an enlarged space a velocity head loss occurs. When pressure is reduced to vapor pressure, localized gaseous conditions occur within a liquid stream. Conversely, Bernoulli's principle provides that fluids entering a reduced area orifice from an enlarged space experience increased velocity. Thus, in a skirted valve, lowered pressure combined with skirt obstructions potentially reduces fluid flow below a desired Cv.
Known methods of addressing the problems with skirted valves include preventing or reducing erosion caused by flashing and cavitations by providing sliding stem angle valves and valves with expanded flow areas downstream of a throttling point because the erosive velocity is reduced. For those areas where the fluid must impact the valve surfaces, such as at the seating surfaces, materials are chosen that are as hard as possible. One known method of preventing cavitation in general is to control the pressure drop across the valve such that the local pressure never drops below the vapor pressure, thereby preventing vapor bubbles from forming. Without vapor bubbles to collapse, there is no cavitation. One known method of controlling the pressure drop across the valve is to split the total pressure drop across the valve using multiple stage trims. These known solutions come at the price of additional expense in further trim requirements, such as additional components and costly materials. Thus, there is a need for a control valve that provides low-noise characteristics while maintaining adequate flow rates for fluids, including gaseous fluids, which have similar noise control requirements.